This past January, Laura Poitras received a curious e-mail from an
anonymous stranger requesting her public encryption key. For almost two
years, Poitras had been working on a documentary about surveillance, and
she occasionally received queries from strangers. She replied to this
one and sent her public key — allowing him or her to send an encrypted
e-mail that only Poitras could open, with her private key — but she
didn’t think much would come of it.

The stranger responded with instructions for creating an even more
secure system to protect their exchanges. Promising sensitive
information, the stranger told Poitras to select long pass phrases that
could withstand a brute-force attack by networked computers. “Assume
that your adversary is capable of a trillion guesses per second,” the
stranger wrote.

Before long, Poitras received an encrypted message that outlined a
number of secret surveillance programs run by the government. She had
heard of one of them but not the others. After describing each program,
the stranger wrote some version of the phrase, “This I can prove.”